1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image. In addition, the present invention also relates to an image forming method, an image forming apparatus and a process cartridge using the toner.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following processes:    (1) charging an image bearing member such as photoreceptors (charging process);    (2) irradiating the charged image bearing member with imagewise light to prepare an electrostatic latent image on the image bearing member (light irradiating process);    (3) developing the electrostatic latent image with a developer including a toner to prepare a toner image on the image bearing member (developing process);    (4) transferring the toner image onto a receiving material such as sheets of paper optionally via an intermediate transfer medium (transfer process); and    (5) fixing the toner image on the receiving material upon application of heat and pressure thereto (fixing process).
Dry developing methods for use in developing an electrostatic latent image are broadly classified into two component developing methods using a two component developer including a toner and a carrier, and one component developing methods using a one component developer including a toner and no carrier. Two component developing methods have an advantage in that images with relatively good image qualities can be produced but have drawbacks in that image qualities change after long repeated use due to deterioration of charging ability of the carrier, change of the ratio (T/C) of the toner (T) to the carrier (C) in a developer, etc.; it is troublesome to control the ratio (T/C) in a developing device; and the image forming apparatus becomes large in size. Therefore, recently one component developing methods, which do not have the above-mentioned drawbacks, attract attention.
In one component developing methods, a toner (i.e., a developer) is typically fed by at least one developing roller to a developing region at which the developing roller faces an image bearing member bearing an electrostatic latent image thereon. The electrostatic latent image is developed with the thus fed toner at the developing region, resulting in formation of a toner image on the image bearing member. In this regard, the toner layer formed on the developing roller is preferably as thin as possible. In particular, when a one component developer (i.e., toner) with a high electric resistance is used, the toner has to be charged in the developing device. Therefore, the toner layer formed on the developing roller has to be extremely thin.
If a strong mechanical stress is applied to the toner in such a thin toner layer forming process, problems in that the external additive of the toner is released from and/or embedded into toner particles of the toner after repeated image formation operations occur, and the surface of the developing roller changes its property or is abraded occur. In these cases, charging properties and fluidity of the toner change, and feeding properties of the toner on the surface of the developing roller change, thereby forming images with low image density or uneven image density and images having streaks.
In addition, when an external additive on toner particles is released therefrom or embedded thereinto, problems in that the toner is adhered to image forming members such as contact chargers tend to be caused. In attempting to solve the above-mentioned problems (releasing and embedding of an external additive, and application of strong stress to toner particles, developing rollers and image bearing members) on the toner side, toners including a porous material as an external additive or an internal additive have been proposed recently.
For example, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 08-137130 discloses a toner including toner particles including a resin having a low softening point as a main component, and a small particulate hollow or porous resin material covering the toner particles. It is described therein that the purpose thereof is to impart a good combination of fixability and durability to the toner. However, as a result of the present inventors' experiments, it is found that the toner particles cannot be fully covered with the particulate hollow or porous resin material when they are mixed using a hybridizer, and therefore the desired effects cannot be fully produced (specifically, the toner has poor durability).
JP-A 06-148931 discloses a toner including a mixture of toner particles and an agent (such as porous carbon) capable of absorbing toxic materials. It is described therein that the toxic material absorbing agent can efficiently absorb ozone adhered to the surface of an image bearing member, and thereby the life of the image bearing member can be prolonged. However, the method for mixing the toner particles with the absorbing agent is not described therein. As a result of the present inventors' experiments, it is found that the particle diameter of the absorbing agent has to be considerably large to absorb ozone. In such a case, the adhesiveness of the absorbing agent to the toner particles is weak, and thereby defective images (such as images with white spots) are formed.
Japanese patent No. 3592501 (i.e., JP-A 11-109824) discloses a technique in that a particulate porous material such as crosslinked polymers having elasticity (e.g., crosslinked polymethyl methacrylate) is supplied to the surface of an image bearing member to reduce the mechanical stresses applied to the toner particles and the developing roller.
JP-A 11-327303 discloses techniques in that an abrasion reducing agent (such as fluorine-containing materials) is included in a toner as an internal or external additive or is coated on the surface of carrier particles; and a porous material (such as molecular sieves (e.g., zeolite)) including such an abrasion reducing agent is used for a toner to reduce abrasion of the image bearing member used. The details (such as specific surface area and particle diameter) of the porous material are not described therein.
JP-A 02-171759 discloses a toner including a porous material having an internal specific surface area of not less than 500 m2/g and JP-A 2000-29237 discloses a technique in that a porous material is added to a developer before the developer starts to be used, to eliminate odor. In order to produce such effect, the porous material has to have a large particle diameter. In that case, the adhesiveness of the porous material to toner particles is low, and thereby defective images (such as white spot images) tend to be formed.
JP-A 2005-17660 discloses a toner including a ring-form polyolefin resin serving as a binder resin (substitute for a polyester resin) and a porous silica with a specific surface area of from 500 to 700 m2/g which serves as an internal additive and which is added to improve the fixability and transparency of the toner. However, the porous silica cannot prevent occurrence of the problems in that the external additive is released from or embedded into toner particles of the toner after repeated image forming operations, and the surface of the developing roller changes its property or is abraded.
Japanese patent No. 2752410 (i.e., JP-A 02-221964) discloses a toner including a porous material, which has a primary particle diameter of from 2 to 15 μm and into which a silicone oil penetrates, to impart good offset resistance to the toner. However, the toner has insufficient charge properties. In addition, Japanese patent No. 3604267 (i.e., JP-A 11-184140) discloses a toner including a particulate inorganic material serving as an external additive which has a porous structure or a high-order structure and includes a release agent in the spaces in the structure, to impart good offset resistance to the toner.
JP-A 2005-241670 discloses a toner including as an external additive a porous titanium oxide having a BET specific surface area of not less than 90 m2/g so that the external additive is hardly released from toner particles and can be easily removed by cleaning even if released from toner particles, resulting in prevention of occurrence of a background development problem. As a result of the present inventors' experiments, it is found that the exemplified porous titanium oxide, which has a specific surface area of from 90 to 100 m2/g, cannot prevent occurrence of the problems in that the external additive is released from or embedded into toner particles of the toner after repeated image forming operations, and the surface of the developing roller changes its property or is abraded.
Because of these reasons, a need exists for a toner which can maintain a good combination of charging property and fluidity and has a long life without being adhered to image forming members (such as chargers) and without damaging the image forming members because the external additive thereof is hardly released from or embedded into toner particles thereof.